In the past, a lay-up of thermoplastic tapes have been pultruded. The tapes were a very coordinated and impregnated series of fiber (such as fiber glass, carbon, aramid, or the like) and fiber bundles that are processed in parallel and fully coated with thermoplastic resin. Example resins used include, but are not limited to, Polypropylene, Polyethylene, and Polyetheretherketone. Example manufacturers of these tapes include, but are not limited to, Polystrand, Inc. of Montrose, Colo., Applied Fiber Systems of Clearwater, Fla., and Suprem AG of Flurligen, Switzerland.
An example composite panel including thermoplastic tape successfully pultruded in the past was 12 inches wide, 0.303 inches thick, and contained 32 layers of Polystrand thermoplastic tape (60% fiber, 40% Polypropylene). In this panel, there were 21 layers of thermoplastic tape in the 0 degree direction, which is the direction of pultrusion (straight out of the die and toward the grippers), and 11 layers of 90 degree material spaced at various locations within the ply schedule.
Although the panel referenced above was successfully pultruded, other types of composite material cross sections present different challenges. For example, there is a need to pultrude very thin sections of composite material that may have only 2, 3, or 4 layers of thermoplastic tape, some of which may be at 0 degree orientation, 45 degree orientation, 90 degree orientation, or any other angle relative to the 0 degree pultrusion direction. These thin materials have the same type of surface friction in the die as a thicker section, but it has been shown that the strength of a 1, 2, 3, or 4 layer composite material pultrusion, where each layer is approximately 0.010 inches thick, is much lower proportionally than a pultrusion of 32 layers of tape, for example. Care must be given to place the die thickness at just the proper gap (known as cavity gap) to achieve excellent pultrusion results. There is much more forgiveness in the die cavity gap in a 32-layer composite material pultrusion at 0.303 inches than in a 4-layer composite material pultrusion at 0.040 inches, for example.
Additionally, there is a need to pultrude thermoplastic tapes over other core materials, creating a sandwich material. As used herein, core materials can include, but are not limited to, wood, metal, ceramics, foam, foam combined with composites, honeycomb, balsa, and any combinations of the same. As examples, sandwich materials can alternate between the thermoplastic tapes and metals that are rigid, the thermoplastic tapes and ceramics, or the thermoplastic tapes and alternating high strength fibers/fiber fabrics to achieve very high ballistic performance. The outside surface of the sandwich panels may need, for example, fabric, film, appliqué, dry paint film, or any other laminating type surfacing material(s). The sum total of these combinations result in an infinite set of combinations of material layers and sandwich thicknesses.
These core materials present their own unique set of challenges. For example, many times these core materials have high compressive strength, and if the thickness is not controlled, a processing problem can occur. In pultrusion sandwich processing, with a variable thickness core, either the core material must compress or, in some cases, crush. If the core material does not crush, the pultruded skin material will jam in the die and the process will be stopped. Often, these core materials are impossible to crush or compress. Accordingly, there is a need for a process that can achieve pultrusion of skins with thermoplastic tape skins and core materials of high compressive strength such as, but not limited to, foam, honeycomb, balsa wood, OSB, and plywood. All of these core materials can be sanded, planed, or shaped to a very tight tolerance, but these processes can be costly. A need exists for a pultrusion system and method for pultruding varying thickness thermoplastic composites with tolerance for variable core thicknesses.
Additionally, a need exists for achieving very high surface finishes and preventing sloughing of 90 degree tape in the die when thin pultrusions are run. Sloughing is the tendency for the 90 degree fiber to arc backwards toward the entrance of the die, and is usually pronounced on the edges of a panel, for example. Sloughing is usually caused by retarding-frictional forces that are encountered in thin panels with only nominal 0-degree layers of tape.